A wide variety of sheet-sensing devices are known for use on machines adapted for the feeding and treatment of sheet lengths. Such devices, generally photoelectric in nature, sense the movement of sheets, such as paper sheets, and provide a signal when each sheet legth enters and/or passes the location of the sensing device, said signal activating or deactivating a work station.
Certain sheet-feeding and treating machines adapted to feed and treat sheets of different lengths, such as certain copying and duplicating machines, are provided with manual switch means whereby the operator activates the switch to signal the machine regarding the length of the next copy sheet to be fed. This activates the machine logic or timing sequence in advance of the sheet movement, whereby the copy sheet is fed into proper position for the formation of duplicate images thereon, corresponding to the location of original images on an original sheet. If the operator neglects to set the switch to the proper sheet length, the machine logic will not correspond to the copy sheet length and the machine will malfunction, in that the timing sequence for each of the work stations in the machine, i.e., electrostatic image formation, image transfer, image fusing, copy discharge, etc., will be longer or shorter than necessary, resulting in defective copies and/or jamming of the paper in the machine and/or a longer period of time to produce a single copy or a succession of copies than is necessary. Also, the possibility of machine break-down and/or fire is increased in cases where the copy paper becomes jammed and/or overexposed, particularly in the heating or fusing station.
It is expensive and impractical to provide machines with sensing devices which sense the length of sheets during passage of such sheets therethrough, particularly in cases where the distance which the sheet travels through the machine is short, as is the case with most office copying machines. Such machines require a more sophisticated logic system, capable of making changes during movement of the copy sheets therethrough. It is far simpler and less expensive to provide a machine with means for sensing the length of sheets before movement of the sheets therethrough but such means must, of necessity, be located in the sheet-supply station. This requirement has presented a major obstacle, up until the present invention, because of the fact that the sheet-supply station must be free of encumbrances which deny the operator easy access to the sheet-receiving areas or which increase the time and/or skill required to supply new sheets to such areas, and also because the sheet length detector is a sensitive device subject to damage if contacted during the frequent sheet-replacement operations.